Default and explicit variance

Research: This page describes research about Eiffel, not the actual language specification.

Introduction

In first the limited variant typing is exposed. It checks all catcall checkpoints. In the second part the mechanism is extended. It is less simple and explicit, but it enables a best transition.

Limited variant typing

What is the advantages of covariance compared to a novariant typing?

• explicit and adaptative interface
• One routine

Variant typing

• A variant entity has the most restrictive type.
• A variant antity requires a simple object test to use the entity with the expected type.

Examples are available in next sections.

Feature redefinition

• Covariant redefinition of request result is allowed (as currently).
• Covariant redefinition of feature argument requires a variant typing on redefined feature or first feature definition.

The most restrictive type for a variant type argument is the type of the first feature definition.

• Contravariant redefinition of feature arguments is allowed, but the type must be the same than the first feature definition or a subtype.

The first rule is a stronger postcondition. The current semantic is not changed. The last rule is a new possibility in Eiffel, it is a weaker precondition.

example 1: covariant redefinition of feature argument with 'variant' typing on redefined feature

Note:

• The object test is not needed.
• the type is not repeated in the object test.
• In the interface of the current type the 'variant' is removed. Indeed, it is not possible to call 'eat' on a COW instance with a parameter of type FOOD.

class
	ANIMAL
 
feature -- Access
	last: FOOD
 
feature -- Eating
	eat (f: like last)
		require
			True
		do
			last := f
		ensure
			True
		end
end

class
	COW
 
inherit
	ANIMAL
		redefine all end
 
feature -- Access
	last: GRASS
 
feature -- Eating
	eat (f: variant like last) -- or eat (f: variant GRASS)
		require else
			True
			-- 'f' type = expected type. Here: GRASS
		do
			-- 'f' type = type of the first definition. Here: FOOD
			if attached f as expected then
				-- 'expected' type = expected type. Here: GRASS
				last := expected
			end
		ensure then
			True
			-- 'f' type = type of the first definition. Here: FOOD
		end
 
end

example 2: covariant redefinition of feature argument with 'variant' typing on first feature definition.

class
	ANIMAL
 
feature -- Access
	last: FOOD
 
feature -- Eating
	eat (f: variant like last)
		do
			if attached f as expected then
				last := expected
			end
		end
end

class
	COW
 
inherit
	ANIMAL
		redefine last end
 
feature -- Access
	last: GRASS
 
end

example 3: contravariant redefinition

note:
	description: eat all foods
class
	MUTANT_COW
		redefine last end
 
feature -- Access
	last: FOOD
 
end

Generic conformance

Default: a generic is novariant

If a genric must be variant (covariant or contravariant or both) then the formal generic must be prefixed with the 'variant' mark.

The compiler must ensure that the variant generic checks one next rule:

• Generic used only on feature argument is contravariant.
• Generic used only on request result or feature argument with variant typing is covariant.
• Generic not used is both contravariant and covariant.

The most restrictive type for a variant generic argument is the constrained inheritance type. example:

deferred class
	EXAMPLE [variant K, variant G]
		-- K is contravariant and G is covariant
 
feature -- Access
	first: G
 
	item (i: K): G
		deferred
		end
 
end

Therefore the next code is valid:

local
	a: EXAMPLE [INTEGER, COMPARABLE]; b: EXAMPLE [NUMERIC, STRING]
do
	-- ...
	a := b
end

Agent conformance

As explained for Usage-site variance the generic 'OPEN_ARGS' from ROUTINE, PROCEDURE, FUNCTION, PREDICATE must be contravariant.

class
	ROUTINE [BASE_TYPE, variant OPEN_ARGS -> detachable TUPLE create default_create end]
-- ...
end

The generic 'OPEN_ARGS' is used on feature arguments, but also on request result.

The class should probably be redesigned. Indeed if contravariant is allowed then there may be a new contravariant catcall on request result.

In these classes there are only two requests using the generic as type:

operands: detachable OPEN_ARGS
 
empty_operands: OPEN_ARGS
	do create Result ensure ... end

The second request is not used internally and may be problematic for creation: How create a tuple of not self-initialized and attached types? The first is used in these features : 'target', 'is_equal', 'set_operands', 'copy', 'apply'

'is_equal', 'set_operands' and 'copy' rely on data model. 'apply' and 'target' are more sensitive. Indeed the problem is when there is an opened target.

'apply' could be restricted for no opened arguments.

apply
	require
		no_operands: open_count = 0
	do
		call (Void)
	end

'target' could be modfied:

target: ANY
	require
		is_target_closed
	do
		-- ...
	end

Or maybe it is better to separate opened target and closed target in two abstractions.

The redesign of agent classes is an opportunity to solve agent problems (see Minor-ECMA-problems, Agents in SCOOP).

Another solution will be mentioned later.

Note: The generic 'RESULT_TYPE' of FUNCTION class is used only on request result. Then it is a covariant generic. It is an expected point.

Comparison with other solutions

Detachable types (non-generic)

The variant typing is not in conflict with the void-safe typing.

And more the object test is not needed.

Usage-site variance (generic)

The variant typing is a supplier specification while usage-site variance is a client specification.

Another difference is the interface restriction of the usage-site variance. With the variant typing for generics the interface is fully aivailable.

Sub-conclusion

The limited variant typing is reasonable and expressive. It limits Eiffel changes and solves all catcall problems.

The proposal reuses an existing keyword.

The addition in TYPE class for reflexivity and dynamic object test is little. Indeed two simple booleans "is_contravariant" and "is_covariant" for each generic is required.

generic_parameter_contravariant (i: INTEGER): BOOLEAN
	-- Is `i'-th generic parameter contravariant?
 
generic_parameter_covariant (i: INTEGER): BOOLEAN
	-- Is `i'-th generic parameter covariant?

A possible critical could be the generic conformance restriction. Propositions are mentioned below.

Extended variant typing

Generic conformance

The variant mark is optional. The compiler infers the generic variance. It is a good point for backward compatibility.

• Generic used only on feature argument or request result with variant typing is contravariant.

Agent conformance

It is possible to declare 'operands' and 'empty_operands' as variant.

operands: variant detachable OPEN_ARGS
 
empty_operands: variant OPEN_ARGS
	do create Result ensure ... end

'apply' and 'target' should be wrote again. For example:

apply
	do
		-- 'operands' type = constrained inheritance. Here: detachable TUPLE
		if attached operands as args then
			-- 'operands' type = request result type. Here: OPEN_ARGS
			call (args)
		else
			call (Void)
		end
	end

Sub-conclusion

This part extend the proposition to enable a best backward compatibility and then a best transition.

General discussions

Greater flexibility for generics

A lot of generics could be novariant, encouraging to propose a solution to have a new flexibility, but safe.

Wildcard generics

The request result type is the constrained inheritance type. And the type of the feature argument is (attached) NONE. The new semantic of 'Void' is considered: Void is not a NONE instance.

local
	a: ARRAY [?]; b: ARRAY [STRING]
	o: ANY
do
	-- ...
	a := b
	o := a.item (1)
	a.put ("try") -- invalid call. The type expected is NONE.
end

However the wildcard generics will be used on request result or feature argument. A more power and elegent solution could be the parametrized routines.

Parametrized routines

do_something [G] (a: ARRAY [G]): G
	require
		a.count >= 1
	do
		Result := a.item (1)
	end

Right abstraction

The wildcard generics and the parametrized routines introduce new constructs for Eiffel. Is there another solution avoiding this?

With a right abstraction it is possible to have flexible classes. It is comparable to the imuutable cocncept.

example: V_CONTAINER class of Eiffel Base 2 To obtain the genric covariance behavior it is necessary to have: - V_ITERATOR must have a covariant generic. - An immutable abstraction of tuple IMMUTABLE_TUPLE should be wrote.

In V_CONTAINER class only two features should use the variant typing:

new_cursor: V_ITERATOR [variant G]
	do ... end
 
occurrences (v: variant G): INTEGER
		do
			-- G -> ANY then 'v' type = ANY
			across Current as it loop
				if it.item = v then
					Result := Result + 1
				end
			end
		end
 
has (v: variant G)
	local
		it: like new_cursor
	do
		-- G -> ANY then 'v' type = ANY
		it := new_cursor -- 'it' type = V_ITERATOR [ANY]
		it.search_forth (v)
		Result := not it.after
	ensure
		occurrences (v) = 1	
	end

In V_ITERATOR only two features should use the variant typing without object test!

With these changes the next code is valid:

local
	a: V_CONTAINER [ANY]; b: V_CONTAINER [STRING]
do
	a := b
 
end

Sub-conclusion

With a right abstraction and a smart use of variant typing for generics, it is possible to obtain a greater flexibility keeping a fully aivailable interface.

Export status restrictions

Since the ECMA Eiffel Standard forbids the export restriction with conforming inheritance, it is not a problem. However, the semantic can be changed to enable this restriction on conforming inheritance.

Restrict exportation should not cause a catcall. The mechanism could be used just to change the class interface.

deferred class
	ANIMAL
feature
	is_vegetarian: BOOLEAN
		deferred end
end

class
	COW
inherit
	ANIMAL
		export {NONE}
			is_vegetarian
		end
feature {NONE}
	is_vegetarian: BOOLEAN = True
end

local
	an_animal: ANIMAL; a_cow: COW
	b: BOOLEAN
do
	create a_cow
	b := a_cow.is_vegetarian -- invalid call
 
	an_animal := a_cow
	b := an_animal .is_vegetarian -- valid call
end

The class interface is more simple and readable.

Conclusion

The proposition uses no new keyword and solves the catcall problem. It passes all Catcall checkpoints.

The new generic conformance gives a natural safe and flexible static typing. The variant typing enables to create adaptive interfaces keeping a safe static typing.

With a fine abstraction and the use of variant typing for generics, the genric conformance flexibility is little restricted. The contravariant for request result can then be discarded for simplicity.

There is no interface restriction (Interval types or Usage-site variance). Class interfaces are fully available.

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